It depends on the operating system and the environment. If its windows 8 the Microsoft built in one is fine, Windows 7 you need to install the Microsoft Security suite. If its Apple I'd just pray, there doesn't seem to be much protection in the Apple world. Android is fairly safe but only use the app store to install new software.
CD, DVD, SSDs and hard drives can now be accessed by Wi-Fi from many tablets by the way depending on the operating system.
Cheerio John